Mark of the Xenos is an excellent addition to the Deathwatch line, and about as close as we'll get (thus far) to a truly comprehensive monster book for the Warhammer 40k RPGs. Pretty much everyone you'd want to kill as a Space Marine is here (except the Eldar and Necrons, sadly), from all the big-name Chaos daemons, to the Tyranids, the Tau's broadside and pathfinders, and the Orkz' boys, nobs and warbosses. If you've got the Deathwatch core rulebook, then you've got the complete set, basically, of tyranid, chaos and tau adversaries for the Jericho Reach, and are unlikely ever to be at a loss for enemies to throw at the Space Marines. What's more, the book is laden with adventure seeds, lore, and fluff ideas for you to base missions and campaigns around, so you can easily take the book and pretty much immediately start running missions out of it. It's fantastic. I just wish that, at some point, we'd get a book that I could review as "completely comprehensive" – with all the iconic adversaries in one place.